In order to enhance spark ablation resistance of a spark plug used for providing ignition in an internal combustion engine, one type of spark plug employs a noble-metal spark portion formed by means of welding a noble metal chip to a distal end of an electrode that is formed from a Ni- or Fe-based heat resistant alloy. The noble metal chip predominantly contains Pt, Ir, or the like.
A method for joining a noble-metal chip to the distal end face of a center electrode to form a spark discharge gap in cooperation with an opposed ground electrode, includes the step of making a full-circled laser-beam weld. For example, see Japanese Patent Application Laid-Open (kokai Nos. H06-45050 and H10-112374). According to the method disclosed therein, a disk like noble metal chip is attached to the distal end face (a chip joint face) of the center electrode, and the boundary between the chip and the electrode is irradiated with a laser beam while the center electrode is being rotated, thereby forming a full-circled laser-beam weld metal portion.
The above-mentioned method for manufacturing a spark plug employs a pulsed laser beam emitted from a YAG laser or the like for welding a noble metal chip to an electrode. To form a laser-beam weld metal portion along the circumference of the noble metal to intrude into the noble metal chip and into a chip joint face formation portion of the electrode, the parameters of the pulsed laser beam such as irradiation energy per pulse and the pulse width are determined while being correlated to the material, outside diameter, and the like of the noble metal chip and electrode. This is done to prevent the formation of a blowhole or the like within the weld metal portion.
When a full-circled laser-beam weld metal portion 10 is formed after a noble metal chip 31′ is attached to an electrode (e.g., a center electrode) 3, a method for attaining a sufficient weld strength between the electrode 3 and the noble metal chip 31′; i.e., a sufficient depth of the weld metal portion, is to irradiate a pulsed laser beam. The pulsed laser-beam is enhanced in laser peak intensity per pulse through reduction of the pulse width, as shown in FIG. 13(a). However, when the weld metal portion 10 intrudes into a chip joint face formation portion of the electrode 3 and into the noble metal chip 31′ as formed by use of a laser beam whose laser peak intensity per pulse is enhanced simply through reduction of the pulse width, spatters (slag and metal particles that spatter during welding) SP are apt to be generated. They are apt to be generated since the amount of heat input by the laser beam is made small as a result of low thermal conductivity of a Ni- or Fe-based heat-resistant alloy used to form the electrode. The thus-generated spatters SP may adhere to the noble metal chip 31′ or the electrode 3 to thereby damage their appearance or impair product yield or in some cases generate a defect such as depression or hole in the electrode.
As the laser peak intensity increases, the weld metal width w of the weld metal portion 10 increases; thus, the distance as measured along the axial direction of the center electrode 3 between the peripheral edge of a discharge face 31a and the corresponding end edge of the weld metal portion 10; i.e., the spark portion thickness h, decreases. As a result, even slight ablation of a noble-metal spark portion 31 is apt to involve exposure of the weld metal portion 10 at the discharge face 31a of the noble-metal spark portion 31. Generally, the weld metal portion 10 is formed from an alloy of a noble metal chip material and an electrode material and is inferior to a sole noble metal chip in terms of spark ablation resistance. Therefore, the progress of ablation of the exposed weld metal portion 10 leads to the expansion of the spark discharge gap in a relatively short period of time. As a result, a problem such as misfire arises; i.e., a problem of impaired durability of the spark portion 31 arises.
When, the laser peak intensity per pulse is reduced by increasing the pulse width as shown in FIG. 13(b) while the total irradiation energy within a predetermined time (e.g., in one second) in FIG. 13(a) is held unchanged, heat input to the electrode 3 tends to be accumulated. But, in some cases the penetration depth d of the weld metal portion 10—which is formed in such a manner as to intrude into a chip joint face formation portion of the electrode 3 and into the noble metal chip 31′—may be insufficient. As a result, the weld strength may be insufficient, or an oxide scale SC (an oxide layer developed in the boundary between the center electrode 3, the noble metal chip 31′, and the weld metal portion 10) may develop in the course of long-term use. The ratio ds/D between the radial depth ds of an oxide scale and the chip diameter D is called the oxide scale development rate. In some cases the noble metal chip 31′ may come off.
In view of the foregoing, an object of the present invention is to provide a method for manufacturing a spark plug which reduces the generation of spatters and the possibility of insufficient weld strength resulting from penetration depth insufficiency of a weld metal portion or the development of an oxide scale resulting from long-term use, as well as to provide a spark plug having high durability of a spark portion which cannot be attained by a conventional method.